Abstract Since the earliest geological investigations on the central Alberta plains, the transition of Upper Cretaceous Montanan deposits from continental beds in the west to marine beds in the east has presented great difficulties in both surface and subsurface mapping. Accelerated petroleum exploration activity in the past 5 years has resulted in a large amount of new subsurface information on which to determine stratigraphical relationships. This paper describes and illustrates with one map and three cross sections, these relationships for the Lea Park and Belly River formations; the younger Bearpaw and Edmonton formations also enter into the discussions insofar as they are involved in the main subject. The interfingering of the marine Lea Park formation with the predominantly non-marine Belly River formation makes possible the recognition of ten members which are somewhat arbitrarily placed in the latter formation. Each of these members has been described previously but most of them have been given more than one name because of uncertainties in correlation between localities. The present study, aided by a greater density of subsurface data, has, it is sincerely believed, succeeded in making sufficiently accurate correlations throughout east-central Alberta to justify discarding several local formation and member names. The correlations are based on lithologic, microfaunal, and elec- tric-log data, although the cross-section illustrations include only the last criteria.

Abstract Oxygen- and carbon-isotope compositions have been determined for clay and carbonate minerals from the Upper Cretaceous clastic rocks of the Milk River and Lea Park Formations. These units contain the Milk River aquifer and the southeastern Alberta Milk River Gas Pool, respectively. The stable isotope data provide important information concerning the diagenesis and paleohydrology of the study area. Authigenic minerals from sandstones in the Milk River aquifer are characterized by low δ 18 O and δ5 13 C values: clay minerals (<2 μm), dominated by authigenic kaolinite, δ 18 O = +11.3 to + 14.2 (SMOW); authigenic calcite, δ5 18 O = +15.3 to +18.5 (SMOW), δ 13 C = −9.9 to −2.6 (PDB). The authigenic minerals with the lowest δ5 18 O values occur within a zone of local recharge in the aquifer. Here the authigenic clay minerals and calcite closely approach isotopic equilibrium with existing meteoric water at low temperatures (<+15°C). Low δ 13 C values forthe calcite indicate incorporation of organically derived CO 2 , probably from decaying plant material in the overlying soil and till. Close agreement between actual formation temperatures and those calculated from isotopic data disappears in downdip portions of the aquifer, mostly because of the drastic enrichment in 18 O of the formation water in this direction (−20 to −6, SMOW; Schwartz et al, 1981). Authigenic minerals from these locations have retained isotopic signatures characteristic of 18 O-poor meteoric water no longer present in the system. This water was displaced by 18 O-rich formation fluids that are themselves now being flushed from the aquifer by modern-day ground water. Authigenic minerals from sandstones in the southeastern Alberta Milk River Gas Pool are more 18 O-rich than those from the aquifer: clay minerals, dominated by illite, +14.7 to +15.8, SMOW; calcite, +19.3, SMOW. Such compositions are compatible with mineral crystallization at low temperatures (+15 to +20° C) from formation fluids similar in δ5 18 O to other Cretaceous oil and gas pools that occur in Alberta. The low- 13 C nature of authigenic calcite and some dolomite (−7.6 to −3.0, PDB) from the Milk River Gas Pool may be related to the production of biogenic methane in this reservoir. Of all clay minerals analyzed, the illite-dominated mixtures from argillaceous rocks of the Milk River aquifer and the Milk River Gas Pool have the highest δ 18 O values (+16.0 to +19.0, SMOW). Such compositions reflect a detrital origin rather than diagenetic processes. Isotopic exchange between these clay minerals and formation water is insignificant. Most dolomite from the sandstones and the argillaceous rocks is not in equilibrium with the authigenic calcite. The dolomite is much richer in 18 O (+24.4 to +28.3, SMOW) and 13 C (−2.7 to +1.0, PDB); such values are typical of platform carbonate rocks. No evidence for extensive isotopic exchange between formation water and the dolomite can be demonstrated.